The present invention relates to a valve characteristic controller and a valve characteristic control method for an internal combustion engine.
Internal combustion engines, such as engines for automobiles, may include a variable valve actuation mechanism for varying the valve characteristic of engine valves including intake and exhaust valves. Examples of valve characteristics include the valve opening and closing timings, the maximum valve lift amount, and the valve actuation angle. In such an internal combustion engine, the variable valve actuation mechanism is hydraulically driven and controlled based on the operation state of the engine so that the valve characteristic of the engine valves is optimized in accordance with the engine operation state. This increases the output of the internal combustion engine and improves the exhaust emissions.
For example, when the internal combustion engine is required to generate a high output, the valve characteristic of the engine valves is adjusted to increase the intake air charging efficiency of the engine. Such adjustment of the valve characteristic increases the amount of air-fuel mixture burned in the combustion chambers of the engine and increases the output of the internal combustion engine.
When the internal combustion engine is not required to generate a high output, the valve characteristic of the engine valves is adjusted to maximize the amount of internal EGR (exhaust gas recirculation) within a range in which the EGR gas does not affect combustion. The internal EGR amount changes in accordance with the valve overlap amount in which the exhaust and intake valves are both open. The valve overlap amount is adjusted to maximize the internal EGR amount in the above range. The maximizing of the internal EGR lowers the combustion temperature and reduces the production of nitrogen oxide (NOx). This improves the exhaust emission of the internal combustion engine.
In internal combustion engines that vary the valve characteristic of the engine valves, there is one type that separately varies the valve characteristic of the intake valves and the valve characteristic of the exhaust valves. Such an internal combustion engine performs the adjustment of the internal EGR amount more accurately so as to raise the output by increasing the intake air charging efficiency and to improve the exhaust emissions. Japanese Laid-Open Patent Publication No. 2003-314308 describes an internal combustion engine including a variable valve actuation mechanism for varying the opening and closing timings of its intake valves and a variable valve actuation mechanism for varying the opening and closing timings of its exhaust valves. This internal combustion engine separately controls the two variable valve actuation mechanisms to vary the opening and closing timings of both intake and exhaust valves.
In this internal combustion engine, the target opening and closing timings of the intake valves and the target opening and closing timings of the exhaust valves are separately calculated in accordance with the engine operation state. The two variable valve actuation mechanisms are controlled so that the opening and closing timings of the intake and exhaust valves are adjusted to the calculated target valve timings. The adjustment of the opening and closing timings of both the intake and exhaust valves in this manner further increases the intake air charging efficiency and improves the accuracy for adjusting the internal EGR amount.
The internal combustion engine described in the above publication controls the valve overlap amount in accordance with the engine operation state so that the valve overlap amount becomes optimum. The internal combustion engine sets the target opening and closing timings of the intake and exhaust valves so as to achieve the optimum valve overlap amount. The opening and closing timings of the intake and exhaust valves are then varied to converge to the target opening and closing timings. By varying the opening and closing timings of the intake and exhaust valves, the valve overlap amount converges on the optimum amount, that is, the target valve overlap amount. The valve overlap amount is optimized in this way.
However, a delay in response occurs when varying the opening and closing timings of the intake and exhaust valves to the target timings. Due to such a delay, the actual valve overlap amount may deviate from the target valve overlap amount when varying the opening and closing timings of the intake and exhaust valves to the target timings. For example, the engine operation state may change when the opening and closing timings of the intake and exhaust valves are converging on the target valve timings. This would accordingly alter the target valve overlap amount. In such a case, the target opening and closing timings of the intake and exhaust valves would also have to change accordingly. As a result, during such transient operation of the engine in which the opening and closing timings of the intake and exhaust valves are still in the process of converging on the target timings, the actual valve overlap amount would deviate from the target valve overlap amount.
To prevent the actual valve overlap amount from deviating from the target valve overlap amount during such transient operation, the above publication proposes execution of a main control and a sub-control to control the opening and closing timings of the intake and exhaust valves.
[Main Control]
The target opening and closing timings of either one of the intake and exhaust valves are calculated in accordance with the engine operation state. The variable valve actuation mechanism for the valves of which target opening and closing timings are calculated is controlled so that the actual opening and closing timings are adjusted to the calculated target timings.
[Sub-Control]
The target opening and closing timings of the other one of the intake valve and the exhaust valve are calculated based on the target valve overlap amount, which is calculated in accordance with the engine operation state, and the actual opening and closing timings of the valves, the opening and closing timings of which have been adjusted by the main control. The variable valve actuation mechanism for the other one of the intake and exhaust valves is then controlled so that its actual opening and closing timings are adjusted to the calculated target timings.
Under the valve timing control including the main control and the sub-control, while the opening and closing timings of either one of the intake and exhaust valves is being adjusted to its target timings, the target opening and closing timings of the other one of the intake and exhaust valves are calculated so as to achieve the target valve overlap amount corresponding to the actual opening and closing timings of the valves of which opening and closing timings are being adjusted. Then, the opening and closing timings of the other one of the intake and exhaust valves is adjusted toward its calculated target opening and closing timing. Accordingly, the opening and closing timings of the other one of the intake and exhaust valves changes so as to achieve the target valve overlap amount. In this manner, the actual valve overlap amount is prevented from deviating from the target valve overlap amount during the above transient operation.
Even if the two variable valve actuation mechanisms for varying the opening and closing timings of the intake valves separately from the opening and closing timings of the exhaust valves have the same structure, the two variable valve actuation mechanisms respond at different speeds when being driven. This is because the two variable valve actuation mechanisms are installed at different positions, and the passages through which oil used to drive these mechanisms have different lengths. This difference in length is reflected as the different response speeds of the two variable valve actuation mechanisms.
Further, one of the two variable valve actuation mechanisms may be driven by hydraulic power and the other one of the variable valve actuation mechanisms may be driven by electromotive power, such as an electric motor. In this case, the two variable valve actuation mechanisms have completely different structures and thus have completely different response speed characteristics. A hydraulic variable valve actuation mechanism tends to have a higher response speed at higher engine speeds at which hydraulic pressure increases. An electromotive variable valve actuation mechanism tends to have a relatively high response speed even at low engine speeds. Such differences in the two variable valve actuation mechanisms having different structures further increases the difference in response speeds.
Regardless of whether the two variable valve actuation mechanisms have the same structure or completely different structures, the difference between the response speeds of the two mechanisms may cause the actual valve overlap amount to greatly exceed the target valve overlap amount during transient operation. Such a state may occur when the main control is applied to the one of the variable valve actuation mechanisms having the higher response speed and the sub-control is applied to the other one of the two variable valve actuation mechanisms having the lower response speed.
In the above state, the reason why the actual valve overlap amount greatly exceeds the target valve overlap amount will be described with reference to the time chart shown in FIG. 6. In section (a) of FIG. 6, the solid line indicates changes in the target opening and closing timings of one of the intake and exhaust valves during execution of the main control. The broken line indicates changes in the actual opening and closing timings of this one of the intake and exhaust valves during execution of the main control. In section (b) of FIG. 6, the solid line indicates changes in the target valve timing of the other one of the intake and exhaust valves during execution of the sub-control. The broken line indicates changes in the actual opening and closing timings of the one of the intake and exhaust valves during execution of the sub-control. In FIG. 6, the arrow formed by a solid line indicates the target overlap amount, and the arrow formed by a broken line indicates the actual valve overlap amount.
The main control is performed with the variable valve actuation mechanism having a higher response speed, which corresponds to the one of the intake and exhaust valves. The sub-control is performed with the variable valve actuation mechanism having a lower response speed, which corresponds to the other one of the intake and exhaust valves. As indicated by the broken line in section (b) of FIG. 6, the main control and the sub-control changes the actual opening and closing timings of the other one of the intake and exhaust valves so that the valve overlap amount decreases, or shifts in an upward direction as viewed in FIG. 6. The valve timing control will now be described.
During transient operation of the engine, the response speed of the variable valve actuation mechanism for the other one of the intake and exhaust valves affects changes in the actual opening and closing timings of the other one of the intake and exhaust valves during execution of the sub-control. More specifically, the actual opening and closing timings of the other one of the intake and exhaust valves changes slowly as compared with the change in the target opening and closing timings. Thus, even if the main control and the sub-control ensure the required valve overlap amount, the actual valve overlap amount (as indicated by the broken-line arrow) greatly exceeds the target valve overlap amount (as indicated by the solid-line arrow) during transient operation of the engine. Further, the actual valve overlap amount also deviates from the target valve overlap amount when the target valve overlap amount changes. However, as shown in section (a) of FIG. 6, when the main control causes the opening and closing timings of the one of the intake and exhaust valves to increase the valve overlap amount, or shifts the valve overlap amount in an upward direction as viewed in FIG. 6, there is a high possibility of the actual valve overlap greatly exceeding the target valve overlap amount.
When the actual valve overlap amount greatly exceeds the target valve overlap amount, the internal EGR amount may become excessive and lower the combustion temperature or increase the blow-by amount of intake air from the intake passage to the exhaust passage thereby destabilizing combustion. This may adversely affect the operation of the internal combustion engine.
The same problem may also occur in internal combustion engines including variable valve actuation mechanisms that adjust the valve overlap amount by varying other valve characteristics, such as the maximum valve lift amount or the valve actuation angle.